PROJECT SUMMARY/ABSTRACT BACKGROUND. Alcohol use (AU) and misuse, including the onset of alcohol use disorder (AUD), peak during adolescence and emerging adulthood. Despite the adverse effects of AU among youth, such as increased risk for the development of substance use disorders and increased risk of alcohol-related deaths, the best available psychosocial interventions for AUD yield only modest short term benefits among youth. One way to improve treatments for AUD and reduce the public health burden associated with youth AU is to advance our understanding of how interventions work. Virtually all evidence-based psychosocial interventions emphasize shifting from alcohol using peers to non-using peers because youth AU predominantly occurs in the presence of peers, and affiliating with AU using peers is consistently associated with increased risk for relapse and worse treatment outcomes. To date, AUD treatment research has largely neglected the peer context of adolescents and emerging adults and this is particularly concerning because failure to consider the centrality of peer relationships in adolescent AU interventions leads to diminished and, in some cases, iatrogenic effects. SPECIFIC AIMS. This F32 supports the first study, to our knowledge, to use ecological momentary assessment (EMA) to examine whether daily level shifts from alcohol using to non-using peers over the course of AUD treatment leads to reductions in drinking and identify when these shifts occur. METHODS. Using an existing data set of a clinical trial examining the effects of medication (topiramate) and motivational enhancement and cognitive behavioral therapies (MET-CBT) relative to placebo plus MET-CBT using EMA (N =82; ages 14-24 years), the project will examine 1) the amount of time youth spend with alcohol-using and non-using peers from pre to post intervention in topiramate and placebo conditions, 2) whether topiramate leads to reductions in AU by attenuating the relationship between being in the presence of alcohol using peers and subsequent AU, and 3) whether lower levels of alcohol use, resulting from the attenuated relationship between being in the presence of alcohol-using peers and subsequent AU in the topiramate condition, in turn, leads to greater time spent with non-alcohol using peers. LONG-TERM GOAL. This research plan provides the opportunity for mentored training in identifying mechanisms of behavior change for youth AUD treatments and advanced training in collecting and analyzing complex EMA data. The identified mentorship team includes experts in the fields of adolescent addiction treatment, mechanisms of behavior change research, clinical trials research, and EMA data collection and analysis., and is housed in internationally recognized clinical and addiction research centers. The current F32 project will facilitate a successful transition to research independence and will support future grant applications that will investigate how social and biological processes impact AUD treatment leveraging EMA designs. Thus, this F32 aids the NIAAA's research priorities of identifying mechanisms of action for pharmacotherapy and reducing youth AU.